This invention relates to devices and methods for measuring and determining the refractive error in eyes and more particularly, a device for calibrating retinoscopes for diverging and/or converging retinoscopy and a method for calibrating retinoscopes using the device.
Retinoscopy is a technique used by practitioners, such as optometrists and ophthalmologists, to obtain an objective measurement of the refractive error of a patient's eyes in order to determine the patient's prescription for spectacles and/or contacts lenses. Varying retinoscopic measurements are frequently obtained with the current retinoscopes. These retinoscopic errors are due the distance between the condensing lens within the retinoscope and filament of the bulb not being standardized and the inability of the examiner to adjust the divergence of the emitted retinoscopy light to their retinoscopic working distance. The divergence of the light rays emitted from all retinoscopes presently on the market is determined by each manufacturer. Subjective refractions using trial frames and phoropters are used as a more accurate alternative for measuring the refractive error. Phoropters are large and expensive pieces of equipment, which means that many practitioners do not have access to them especially in third world countries. Subjective refractions require a feedback from the patient as to whether one lens is clearer than another lens in order to obtain an accurate measurement of the refractive error. Since subjective refractions cannot be used on patients, such as infants, small children, foreign speaking patients and deaf and/mute patients, an objective measurement using retinoscopy is required. Auto-refractors are used frequently in industrialized countries to replace retinoscopy; however the results are inaccurate in many cases, especially in older patients. As a result, many of these patients are dispensed incorrect lenses.
Jack Copeland was the originator of streak retinoscopy as practiced and taught today and designed the Copeland Optec 360 Streak Retinoscope. The technique of streak retinoscopy is illustrated in Videotape No. 5063 for the American Academy of Ophthalmology's Continuing Ophthalmic Video Education series, entitled “Retinoscopy; Plus Cylinder Technique.” U.S. Pat. No. 3,597,051 of Copeland illustrates his streak retinoscope assembly which has a thumb-slide adapted to move the power capsule housing the bulb and battery up and down relative to the shaft of the retinoscope. The ridges on the knurl on the power capsule allow one to rotate the power capsule housing the bulb 360°. Advancing the thumb-slide to its upper position causes light rays emanating from the retinoscope to diverge. By contrast convergence of the light rays occurs when thumb-slide is moved to it lower position. The Copeland Optec 360 Streak Retinoscope contains a +20.00 D condensing lens and a bi-pin lamp. When the thumb-slide is in its upper position, the filament of the lamp is less than five centimeters from the condensing lens and the rays emanating from the filament and passing through the condensing lens are diverging. Moving the slide to a lower position causes the light rays to converge. The filaments of the light bulbs for the Copeland Optec 360 Streak vary in height which results in differences of the divergence power of the emitted retinoscopic light ranging from 0.00 D to 1.00 D. Many retinoscopes on the market work in the opposite manner as the Copeland Optec 360 in that converging rays are produced when the knurl is in the up position and diverging rays when the knurl is in the down position.
In retinoscopy, the examiner uses a retinoscope to shine light into the patient's eye and observes the retinal reflection which is referred to as a pupillary reflex. While moving the streak of light across the pupil the examiner observes the relative movement of the pupillary reflex as they use a phoropter or manually placed lenses over the eye to “neutralize” the pupillary reflex. Streak retinoscopy uses three images to measure the refractive state of the eye. The first image, Image I1 is the luminous streak of the retinoscopic bulb. The second image, Image I2, is the focus or non-focus of image I1 onto the reflecting membrane of the retina. The reflection of the Image I2 produces a third image, Image I3, the pupillary reflex. The non-focus light of Image I1 on the iris is the intercept. The examiner can only see the intercept and the Image I3. The examiner draws all signals from the intercept and third image, Image I3, as when to rotate the pupillary reflex to align the astigmatic axis and to add or subtract lenses (+ or −) to neutralize a refractive error. The retinoscopic working distance is the distance between the luminous filament of the light bulb from the pupillary plane of the eye. Clinically, the retinoscopic working distance in conventional retinoscopy is manually measured with a string from the examiner's nose to the spectacle lens plane in the phoropter or trial frame. The principals of spot retinoscopy are the same as streak retinoscopy; the only difference is the pupillary streak is a spot of light instead of a streak. Streak retinoscopy is popular in the United States, South America and Canada and spot retinoscopy in the European countries.
In retinoscopy, upon neutralization of the refractive error, the diverging retinoscopic light rays used in streak and spot retinoscopy are focused into the eye with a spherical lens placed in front of the eye. This spherical lens is called a retinoscopic spherical lens or fogging lens and its focal length is equal to the examiner's retinoscopic working distance. A neutrality reflex, indicating neutralization of the refractive error, occurs when emitted retinoscopic light exits the retinoscopic spherical lens and enters the pupil as parallel light rays and is focused onto the reflecting membrane of the retina. The reflected light then exits the eye as parallel light rays and is focused by the retinoscopic spherical lens into the hole of a mirror or surface of a semi-reflecting mirror within the retinoscope. This endpoint is called “gross retinoscopy.” “Net retinoscopy” occurs on removing the retinoscopic spherical lens, thereby allowing the patient's visual focal point to be extended from the mirror to the end of the refracting lane. If the focal lengths of the retinoscopic working distance, the retinoscopic spherical lens and the emitted diverging retinoscopic light are not equal, the pupillary image will be focused in front or behind the mirror, creating myopic and hyperopic retinoscopic errors respectively. When the pupillary reflex is refocused to the hole in the mirror by the retinoscopist to obtain an infinity pupillary reflex, myopic and hyperopic retinoscopic errors are created.
The variability of conventional retinoscopy is due to several factors, which include the exponentially expanding and moving pupillary reflex which becomes infinite and cannot be seen at neutralization of the patient's refractive error, the low luminosity of the pupillary reflex created by the diverging retinoscopic light which is not calibrated to the examiner's retinoscopic working distance and/or the focal length of the retinoscopic spherical lens. Accommodation and pupillary constriction induced by the retinoscopic light, whether on or off-axis, further reduce the illumination of the pupillary reflex. Retinoscopy through dilated pupils induces optical aberrations and peripheral movements that are different and more myopic than the central pupillary reflex. These factors make it difficult to recognize a definitive endpoint and have contributed to the variability and inaccuracy of conventional retinoscopy.
Retinoscopists use a meridional straddle to confirm the accuracy of the retinoscopic endpoint in conventional retinoscopy. The quote below is from the editorial staff of the American Academy of Pediatrics and Strabismus: “In the 25 years that I taught Retinoscopy and in the 45 years that Copeland taught the subject, calibration of the Scope was to be avoided. Why? Because the endpoint did not depend on calibration of the instrument but rather meridional comparison and ability to utilize all of the various steps together to get to the working distance endpoint.”
The reason that an accurate meridional straddle with conventional retinoscopy is important is due to the fact that the endpoint image signifying neutralization of the refractive error cannot be seen when focused into the hole in the mirror or mirror. A meridional balance is when the myopic pupillary reflex is under corrected 0.50 D to create a hyperopic pupillary reflex, which is easily seen and then overcorrected 0.50 D to create a myopic reflex. This process is repeated several times during a retinoscopic examination. Most retinoscopists use the hyperopic reflex as their endpoint which under-corrects the retinoscopic endpoint. As previously described myopic and hyperopic retinoscopic errors occur when the divergence power of the emitted retinoscopic light is not neutralized by the power of the retinoscopic spherical lens and the retinoscopic working distance not being equal to the focal length of the retinoscopic spherical lens. If these three variables are not equal, the retinoscopic endpoint is focused in front of the mirror or beyond the mirror in which case the meridional balance only confirms the accuracy or inaccuracy of the retinoscopic endpoint. The accuracy of the spherical retinoscopic measurement is dependent on the three variables being equal. Statistically, the inaccuracy of the spherical retinoscopic measurement as compared to the measurements of the cylinder and axis supports this theory.
To improve the endpoints of conventional retinoscopy and eliminate other optical problems of conventional retinoscopy Boeder and Kolder developed a retinoscopic technique using parallel light rays emanating from the retinoscope and claimed this produced “neutralization at infinity” or the ability of the patient to read the Snellen chart during retinoscopy. In their formula describing neutralization at infinity, emmetropia or RSR=1 was achieved when the relative speed of movement of the intercept and pupillary reflex were equal and the pupillary reflex was no less than 2.0 mm. This is expressed in the following formula:RSR=[1−t/I1]/[1+t/I3]=1                RSR=relative speed of pupillary reflex        I1=focal length of Image I1         I3=focal length of Image I3         t=retinoscopic distance (cm)        Image I1=luminous filament of the bulb        Image I3=pupillary reflex is the reflection of the retinoscopic light        
Unfortunately, their technique required the retinoscopist to recognize the width of a 2.5 mm with-motion pupillary streak as the endpoint, or one no less than 2 mm, an unfamiliar unit of measurement to retinoscopists and many of their claims are invalid:                1. The premise that their infinity retinoscopic technique placed the endpoint of neutralization at infinity instead of the aperture of the retinoscope.        2. The intercept and pupillary reflex will move at different speeds if infinity neutralization is not achieved. That is, if I3 is on the myopic side of neutralization, the with-motion pupillary reflex I3 moves slightly ahead of the intercept and slower if I3 is on the hyperopic side of neutralization.        3. At infinity neutralization, the power of Image I3 is independent of the retinoscopic working distance.        4. The pupillary reflex is characterized by “well defined borders” and “evenly distributed brightness” only at neutralization.        5. The endpoint of infinity retinoscopy or emmetropia occurs only when I1=I3 is incorrect since in converging infinity retinoscopy, I1 can be greater or less or equal to I3 when emmetropia is obtained.        6. Naming the article “Neutralization at Infinity in Streak Retinoscopy” since neutralization occurs at the examiner's retinoscopic working distance, not infinity.        7. In the derivation of the infinity formula, Image I1 and I3 are located at the position where the retinoscope is located, not beyond the retinoscope and this makes RSR=∞, not 1, when parallel light rays are emitted from the retinoscope.        8. The formula RSR=[1−t/I1]/[1+t/I3]=1 does not apply to parallel light emitted from the retinoscope, since the formula is derived on the assumption that the focal lengths I1 and I3 will be greater than t, the retinoscopic working distance. In contrast, with parallel light emitted from the retinoscope, RSR is equal to 1 and t is equal to the focal length of I1 and I3.        
On an inter-national scale, the average optical remake of spectacle lenses is 6-10%. These lenses are remade at no cost to the patient. This extrapolates to a financial and efficiency loss to the physician and optical shop as well as a reduction of the patient's opinion of the professional skills obtained in that office. The optical remakes and complaints have not been reduced by the auto-refractors or the present day refraction and/or retinoscopic techniques.
Therefore, a need exists for a device and method that eliminates many of the factors responsible for the variability and retinoscopic errors of conventional retinoscopy. More specifically, a need exists for a calibration technique calibrating a retinoscope to the examiner's retinoscopic working distance using either converging or diverging light emanating from the retinoscope.
The relevant prior art includes the following references (U.S. unless stated otherwise):
Pat. No.InventorIssue/Publication Date5,650,839SimsJul. 22, 19975,500,698SimsMar. 19, 19965,430,508SimsJul. 4, 19953,597,051CopelandAug. 3, 1971